Transcutaneous electrical nerve stimulation (T.E.N.S.) is a procedure in which an electrical signal is transmitted through the skin to appropriate underlying nerves where it helps prevent pain signals from reaching the brain. T.E.N.S. has been used for about ten years in major hospitals, medical schools and pain clinics with good results. It is a very desirable method of pain control because it is safe, effective, non-invasive, non-narcotic and has no known side effects.
T.E.N.S. requires the placement of an electrode on the body at the site of stimulation to deliver the electrical signal to the skin beneath the electrode. Typically, the electrode will consist of a connector having means for attachment of a lead wire and a relatively flat, body-conformable, electrical impulse-distribution portion. Metallic foils or conductive plastics are commonly used to fabricate the connector. The connector is attached via the lead wire to a stimulator. To enhance electrical conductivity between the connector and the skin and to reduce the likelihood of "hot spots" developing, a conformable, electrically-conductive interface material is applied between the skin and the connector. The traditional interface material is a conductive gel or a pad saturated with a conductive gel. The gel or gel pad is applied to the skin and overlaid with the connector. An adhesive patch is generally required to hold the electrode in place. This type of electrode assembly suffers from a number of disadvantages. Firstly, conductive gels are predominately water and tend to dry out within a relatively short time after application to the skin and lose their conductivity. This necessitates removal of the electrode and re-application of gel on a daily basis. Secondly, the skin often becomes irritated from the repeated removal of the adhesive patch holding the electrode in place.
These problems were alleviated somewhat by replacing gel with a dry, electrically-conductive interface material, such as plasticized karaya gum, which does not dry out during use. However, karaya gum pads have low adhesion, and an adhesive patch is sometimes required to hold the electrode assembly on the skin. The adhesive patch restricts the movement of the underlying skin and causes a great deal of discomfort to the wearer over prolonged periods of use. Since T.E.N.S. is used for the management of chronic pain, prolonged application of an electrode, e.g., two to seven days, is desirable.
The need has developed for a disposable T.E.N.S. electrode which (1) is economical; (2) does not dry out during use; (3) is self-adhering; and (4) is extensible with the skin.
The prior art has provided T.E.N.S. electrodes meeting one or more of the aforementioned criteria, but has been unsuccessful in providing an electrode meeting all of them. For example, U.S. Pat. Nos. 4,008,721, 4,067,342 and 4,141,366 disclose dry, self-adhering stimulating electrodes wherein the interface layers are pressure-sensitive adhesive materials rendered conductive by the inclusion of conductive metallic particles. These electrodes contain non-woven fabric backings which limit the ability of the electrodes to move with the skin.
U.S. Pat. No. 4,066,078 discloses stimulating electrodes wherein the interface material is an electrically-conductive adhesive comprising certain organic polymers plasticized with a polyhydric alcohol. In the electrode described in U.S. Pat. No., 4,125,110, the conductive interface material is a mixture of a natural organic polysaccharide, such as karaya gum, and a polyhydric alcohol. The interfacing layers in the aforementioned two electrodes are bonded to a metallic foil. The foil is not extensible and restricts the extensibility of the interfacing layer. Furthermore, the interfacing layer in both electrodes is quite thick and too costly to provide a practical, low-cost disposable T.E.N.S. electrode.
The T.E.N.S. electrode of the present invention provides a combination of desirable features heretofore unavailable. In addition to having a "dry" conductive interface layer, which can be used for a period of days without drying out, the thickness of the layer is kept at a minimum to provide a low-cost, disposable electrode. The unique backing layer of the electrode, in combination with the interface layer and the low profile connector, provide an extensible electrode that stretches with the skin and can be worn for long periods, without discomfort or irritation, even over skin areas undergoing constant stretching, e.g., lower back.